THE WOMAN sitting opposite me smells faintly of Lentheric. It is hardly the scent of sex, but it's what her mother wore. I shall call her Alison, though that is not her real name. She is 48. Her flatties are scuffed, and she flicks away the tabby that is scratching the sofa.

For 14 years Alison worked as a secretary to a Tory MP who is married with children. His is a household name, for he was once the mainstay of one of the parliamentary committees. For eight of those years, he and Alison were lovers. Alison does not look like an adulteress. But 'that is what I was,' she says. 'Funny word, adulteress. It sounds so sinful.

'His constituency was far from London. His wife stayed up there mostly. But he was in London from Monday to Thursday when Parliament was in session. He probably saw more of me than of her. You know I never noticed it at the time, but I looked quite like her.

'The fact that they were apart so much wasn't a problem at first. The children were growing up. She was busy. He was keen to get on, and there was so much to do down here. I was terribly involved in his work. I loved the job. I thought what he was doing was really important.

'Then one day, we'd been working late. You know MPs' days are very long, what with late-night sittings and all that.' It sounds so simple; so slippingly easy you almost mouth the words with her. 'He seemed a bit down. He said he was really lonely. I remember he looked so little, I just put my arms around him. I felt so sorry for him. That's when it began, really. It just sort of happened.' The tabby cat flexes its claws. She ignores him.

'He didn't have much money, so he couldn't give me any. I didn't mind. I mean he'd pay if we went out for a meal. But I wasn't kept or anything.'

Kept or not, Alison was a mistress. For eight years, she chose to stay devoted to this man, forsaking marriage and a family of her own, putting up with empty weekends and lonely Christmases. 'I don't know whether his wife knew about it, but if she did she turned a blind eye. Maybe she knew I would always look after his best interests; I wouldn't say anything.'

The affair ended when Alison resigned because she realised he would never leave his wife. While it lasted she feasted on the fantasy of their next few hours alone together and the conviction that somehow, in this hopeless entanglement, she was doing something important.

Alison is one of the many mistresses who exist, some quietly and others not, in the mother of parliaments. That they exist at all is no great surprise. After all, an MP's life is hard. The hours are very long. People are thrown together, and sometimes they will fall in love. Lord Lawson's second wife was once his secretary. So was Douglas Hurd's.

Others pay a high price for relationships formed at Westminster. Either the MP pulls himself together and his pants up (not always happily), or he gets caught. Then it's curtains. The list of members whose careers have been cut short is a long one. Cecil Parkinson is the best known, and David Mellor may soon be joining him , but there are others as well.

IF you are a married MP and considering an affair, the rules of engagement are simple: don't get caught. Say nothing to anyone. Be beyond reproach. If asked about it by reporters, David Mellor told Antonia de Sancha, 'put the phone down.'

Ambitious MPs are never seen with their lovers in public, though one or two might risk those little hidden-away places south of the river. For the less careful, the coyly named Italian basement eatery L'Amico, in the Horseferry Road, is a favoured place, so is Green's in Marsham Street or La Poule Au Pot in nearby Ebury Street. Some even invite their mistresses into the House for drinks in the Terrace bar overlooking the river. These MPs are either brazen or thick.

Why do they do it? What makes them so incredibly arrogant as to think they can get away with it? 'MPs are incredibly highly sexed,' suggests Angie Bray, who works for the commercial parliamentary lobby firm, Ian Greer Associates. Sarah, the wife of a new Tory member, agrees. 'It's got to do with the fact that they're performers. They perform in meetings, they perform in front of crowds, they're charming, persuasive, amusing. They're good talkers. They've got to be or they'd never get elected.'

Her husband, the new MP, chimes in: 'There's something about political life, public life, that makes people sail really close to the wind.'

This isn't true of women MPs, who seem to take the job more seriously. But it is of the men. 'There's enormous pressure to conform, to sell yourself as something more than you are, to put on an act. All those TV cameras, constituents, receptions. You're always pretending you're a grown-up.

'There's got to be scope somewhere to really let your hair down.'

THE biggest question asked about Mr Mellor after the story of his alleged affair broke last week was not is it true, but how could she. Mr Mellor is a clever man, but he is no oil painting. Neither are most MPs. Yet clearly there's something irresistible about them. Perhaps they are more libidinous.

'If you go to a dinner party and there's an MP there,' says Marilyn, a House of Commons researcher, 'he'll probably be the only one. And he'll be the star of the evening. It's a catch having an MP at your table. He'll be really good at turning on the attention. He'll listen to the women, and tell stories to the men. I know that probably sounds really sexist. But that's what it's like.

'These people are where the power is. They do all the stuff that gets in the papers. They bang on about Maastricht or Prime Minister's Questions. It all rubs off on them. Even the ugliest one's got something.'

Marilyn's sister has nothing to do with the House, but even she notices it. 'I was at a drinks party recently. The hostess came up and took me by the elbow. 'There's someone I want you to meet.' It was Paddy Ashdown. He was gorgeous. For five minutes, he made me feel I was the most important person there.

'But then I noticed that every 10 minutes or so she'd bring over another girl. It was always girls; never men. There was no hanky panky . But he was the same with all of them. Really turned on the charm. He got my vote.'

IF girls are what it takes to unwind, then there's no problem meeting them. Mr Mellor was introduced to Ms de Sancha by a Private Eye journalist. Junior MPs are invited to three or four receptions a night. 'British Airways even lays on air hostesses to pass round the drinks,' says one. And there is no shortage of attractive women in the House. All MPs have access to a secretary, most of whom are female. So are many of the researchers, who, like the secretaries, are employed by the members and not by the House.

'The researchers are almost always young,' says Andrew Gifford, director of another lobbying firm, GJW Government Relations. 'They're relatives, friends, friends of relatives, exchange researchers over from America. They're paid a pittance; sometimes nothing. Often, they're only there for a short stint, so they're out for a good time. Oh yes, there's plenty of naughtiness with the researchers,' he adds with a chuckle. 'Not to mention what goes on at party conferences.'

This is a florid view of the House. For every playful MP, secretary or researcher, many have their eyes on nothing more than the job in hand. Nevertheless, the view of Westminster as a palace of promiscuity is shared by Caroline, who was once a Commons secretary until she married a Tory MP. 'There's lots of sex in the House. Tons of it. You know straight away which MPs are getting up to it and which aren't. They chat you up, and try to impress. It's all flattering stuff, and if you're tempted you go for it.

'I know two former MPs who used to hunt together as a pack although they were in different parties. I won't tell you who they are but they'll recognise themselves. They would even go outside the House to the queue of visitors waiting to get in. And if they saw a pretty girl, they'd offer to take her round. Just like that.'

It is easier for an MP to have an affair with someone working in the House than an outsider. MPs work with their secretaries or researchers. Gossip is containable. The curious system of the Lobby, which forbids journalists from citing their sources and, sometimes, even stops them repeating some of what they hear in the House, means a surprisingly small proportion of Commons gossip gets into the newspapers.

Members' wives may never learn of their husbands'affairs. Among those who do, some choose to say nothing. They may silently ally themselves with the mistress if they feel the MP is protected. Alison believes her lover's wife did this. 'Better me than a dolly bird. I would always be loyal.' Others consider divorce, but the price is high. Silvia Rodgers, wife of the former SDP vice- president, once wrote: 'The opprobrium for walking away is greater, and the damage caused can extend to the Party or the Government.'

A Commons affair is not difficult to schedule. In the morning, MPs work mostly on constituency business in their rooms. Or they've got committee meetings. Questions start at 2.30. On Tuesdays and Thursdays at 3.15 is Prime Minister's Question Time. And it goes on. They float in and out. They go to the bar, or have a bite to eat. The vote isn't until 10pm. 'Ring up the Whips Office any night when your husband is not at home and with the utmost discretion they will always reassure you that they saw him only a minute ago,' says Silvia Rodgers. But, adds Caroline: 'MPs are awfully difficult to pin down during the day.'

If MPs' hours facilitate fornication, the hothouse environment of Westminster condones or even encourages it. 'Wesminster is the ultimate insiders' club,' says Caroline, clearly chuffed to be in on it. 'They all know each other. They're all on the make. And they gossip all the time.'

Like all clubs, the Commons is profoundly arrogant. The Lobby system lulls MPs into the belief that they will never be caught. The rules are designed to protect the members. But they also help them forget (temporarily) that they have made commitments - to wife and family, not to mention Queen and country. 'They're like children in a sweet shop,' says Sarah. 'A lot of them are just stupid men who can't control their cocks.'

David Mellor got caught. Alison and her lover never did. Maybe she wishes they had. Then she would have rebuilt her life earlier. Sarah knows her husband is more attractive than most MPs. She trusts him. Even so, she's taking no chances. She has got rid of the secretary who worked for her husband's predecessor, and plans to take on the job herself. 'I'm aware of the potential for problems. I'm aware that women will be tipping their cap at him. But if I control the diary, take the telephone calls and am seen around with him, I think we'll be OK.'

Sarah has two children under the age of four and she knows the job will take it out of her. But she reckons she has to do it. 'I've got to make sure he never has it so good anywhere else.'